1. Field of the Invention
This invention relates in general to spindle motors for use in magnetic disc storage systems. More particularly, this invention relates to magnetic disc storage systems having spindle motors that use hydrodynamic bearings.
2. Description of Related Art
Data storage systems, such as disk drives, commonly make use of rotating storage disks. The storage disks are commonly magnetic disks but could also be optical. In a typical magnetic disk drive, a magnetic disk rotates at high speed and a transducing head uses air pressure to "fly" over the top surface of the disk. The transducing head records information on the disk surface by impressing a magnetic field on the disk. Information is read back using the head by detecting magnetization of the disk surface. The magnetic disk surface is divided in a plurality of concentric tracks. By moving the transducing head radially across the surface of the disk, the transducing head can read information from or write information to different tracks of the magnetic disk.
Spindle motors are commonly used to rotate magnetic disks at high speeds. Frequently, conventional spindle motors comprise small electric motors equipped with standard ball bearings. However, electric motors having ball bearings are known to experience problems such as runout or vibration that can prevent information from being accessed from disks rotated by the motors. This is especially true as advancements in data storage technology have increased magnetic disk storage densities. To overcome the problems associated with ball bearing electric motors, some disk drive systems now make use of electric motors having fluid hydrodynamic bearings. Bearings of this type are shown in U.S. Pat. No. 5,427,546 to Hensel, U.S. Pat. No. 5,516,212 to Titcomb and U.S. Pat. No. 5,707,154 to Ichiyama.
An exemplary hydrodynamic bearing typically includes a stationary shaft on which is mounted a rotary hub to which magnetic disks can be secured. There is no direct contact between the rotating hub and the shaft. Instead, a lubricating fluid such as air or oil forms a hydrodynamic bearing between the shaft and the rotary hub. Hydrodynamic pressure or pumping is frequently provided by a pattern of grooves, commonly in a herringbone configuration, defined either by the exterior surface of the shaft or the interior surface of the rotary hub. During rotation of the hub, the pattern of grooves provides sufficient hydrodynamic pressure to cause the lubricating fluid to act as a bearing between the shaft and the rotary hub. Frequently, capillary seals are used to retain the bearing fluid between the shaft and the rotary hub.
When used in association with spindle motors, air bearings provide numerous advantages. For example, air bearings are more efficient and consume less power than either ball bearings or oil bearings. Also, air bearings are quiet and have excellent run out characteristics. Air bearings also have disadvantages. For example, when air bearings are used in disk drive spindle motors, it can be difficult or expensive to simultaneously provide both thrust (e.g. axial) and journal (e.g. radial) bearing support. Also, sliding friction associated with thrust operations during motor start-up and shut-down can create wear debris that reduces the efficiency of the motor. Additionally, air bearings often require more space than either ball or oil bearings thereby providing less space for the motor. Finally, air bearings are typically not effective for low rotational speed applications.
Oil bearings also have advantages when applied to disk drive spindle motors. For example, oil bearings are generally quiet and have good run out characteristics. Also, oil bearings occupy less space than either ball bearings or air bearings. However, oil bearings also have disadvantages. For example, oil bearings consume more power than ball bearings or air bearings. Furthermore, when oil bearings are used in the journal bearing environment, oil leakage can be problematic.
In the future, spindle motor disk rotation speeds will steadily increase. As disk rotation speeds increase, the problems associated with standard oil bearings, air bearings and ball bearings will become magnified. Increased disk recording density is another trend in the industry. The combination of increased disk rotation speeds and increased recording densities will require disk drives to operate with improved run out characteristics. What is needed is a disk drive bearing system that has excellent run out characteristics, that demonstrates long life even when used at high rotational speeds, that eliminates oil leaks, that reduces wear and friction, that generates power savings, and that is relatively efficient to manufacture.